Municipal wastewater contains a lot of different substances. The most commonly used parameters comprise biochemical oxygen demand (BOD), suspended solids (SS), phosphorus (P) and nitrogen (N). These parameters are typically regulated by authorities.
Municipal wastewater treatment processes typically include several steps, all designed to provide water that is sufficiently clean for returning to water streams. These process steps may include                mechanical, optionally with added inorganic chemical, designed to separate suspended solids and possibly phosphorus from raw sewage for example in sedimentation basins,        biological, designed to consume organic matter using microbes, preferably followed by a further sedimentation step designed to further separate suspended solids, and        nutrient removal, which can be a part of the biological process or done by chemical treatment.        
In the mechanical treatment, typically about 30% of the BOD and 50-60% of the SS is removed and very little of P and N. In many cases this is not enough and a chemical is introduced to improve the separation. The chemical added may be an iron or aluminium salt. These salts are normally called coagulants or inorganic coagulants and will support the particle, BOD and phosphorus removal. Up to 95% of SS and 75% of BOD can be reduced if a coagulant is added to the process simultaneously with the mechanical treatment. Since iron and aluminium also precipitate phosphorus, dissolved phosphorus from the wastewater is also separated when inorganic coagulants are used. Up to 98% P can be reduced. In some cases a chemical enhanced primary treatment (CEPT) is enough to fulfil the wastewater effluent standard. In that case this process is described as a CEPT plant or a direct precipitation plant.
In the biological treatment, remaining SS and BOD is removed. In this step natural bacteria degrades organic substances to biomass, carbon dioxide and water. This is normally done under aerobic conditions where oxygen is added to the water to increase the efficiency, for example in an activated sludge process. Biological treatment can be completed by simultaneous phosphorus reduction process in which chemical phosphorus precipitation takes place at the same time as biological treatment in an activated sludge process. The biological stage also serves as a coagulation basin, and both the biological sludge and the chemical sludge are removed in the subsequent stage. The coagulants that are normally used for simultaneous precipitation are aluminium salts or iron salts.
The above described combined chemical and biological wastewater treatment plant is efficient in reducing the amounts of suspended solids, biodegradable organics, and nutrients, including nitrogen and phosphorus. However, particularly in the case of the nutrients, there is still a need for a process that will recover these nutrients efficiently, while still reducing the amounts of suspended solids and organics, whereby said nutrients can be separated, recycled and reused.
Wastewaters may contain phosphorus in the form of orthophosphates, organic bound phosphorus and polyphosphates. The concentrations of phosphorus in municipal wastewaters may typically be for example 3-20 mg/l, but also much higher concentrations are possible. Some industrial wastewaters may contain 100 mg/l or even higher amounts of phosphorus.
The removal of phosphorus from wastewaters is an essential step of wastewater treatment, due to the role that phosphorus has in eutrophication. Phosphorus removal is therefore typically regulated by authorities.
Phosphorus in wastewater exists in particulate and dissolved matter. The dissolved matter mainly consists of phosphates. In a WWTP, it is therefore essential to remove both particulate and dissolved P and this can be done in different ways by chemical or biological means.
Chemicals for phosphorus removal, such as inorganic coagulants, can be applied in different parts of a WWTP. Phosphorus can be separated from wastewater in a direct precipitation plant with no further treatment. Phosphorus can also be precipitated in an activated sludge tank. This process is called simultaneous precipitation or co-precipitation. The precipitated phosphorus remains mixed with the sludge.
Biological phosphorus removal may be done by two different means, assimilation and enhanced biological phosphorus removal. Assimilation is the uptake of phosphorus in the biomass. This is done in all living organisms but the amount of phosphorus that is taken up is not so high. Only about 1% of the biomass is phosphorus. So with more biomass production in biological treatment more phosphorus can be taken up. With chemical pre-treatment it is possible to control the BOD load to the biological treatment. With a high BOD load more phosphorus can be assimilated and vice versa.
Enhanced biological phosphorus removal (EBPR) is an activated sludge process where bacteria with higher potential to take up phosphorus are encouraged to grow. In this process the bacteria both release and take up phosphorus. Under anaerobic conditions phosphorus is released and under aerobic phosphorus is taken up. This means that if the biological sludge is anaerobically digested to produce biogas, some phosphate is released. This phosphate release can cause scaling problems in the digester and the dewatering system after the digester but can also open up for recovery of some phosphorus. EBPR processes are not as efficient and stable as chemical phosphorus removal process and are normally backed up with a chemical process to ensure that effluent requirements are fulfilled.
Chemical phosphorus removal has a high efficiency of removing phosphorus from wastewater and precipitating it into the sludge. It can then be recycled to the agricultural land with the sludge and used as a nutrient. On the other hand, the biological phosphorus removal process is less efficient and the wastewater contains typically higher amounts of phosphorus. However, phosphorus can be recovered in the side stream after the anaerobic digestion that can release phosphorus. This is, however, an internal recycle stream at the plant with low phosphorus removal efficiency and only small portion of phosphorus is recovered i.e. 5-30% of the influent phosphorus.
There is still a need for improved procedures that efficiently concentrate and separate the phosphorus compounds, so that they can be efficiently recovered for further use.